1. Field of the Invention
The present invention relates to a device for purifying exhaust gas of an internal combustion engine featuring an improved NOx purifying efficiency even in a lean operating condition without deteriorating the fuel efficiency, as a result of using an electrochemical catalyst as a catalyst for purifying harmful gases (NOx).
2. Prior Art
The exhaust pipe of an internal combustion engine is usually provided with a device for purifying exhaust gas in order to purify harmful components (NOx, HC, CO, etc.) contained in the exhaust gas.
The three-way catalyst that has heretofore been extensively used works to react NOx (nitrogen oxides) generated in the stoichiometric air-fuel ratio mode operation, HC (hydrocarbons) which are the unburned fuel components, and CO (carbon monoxide) which is a component produced by the incomplete combustion, simultaneously, in order to decrease the amount of emission of NOx, HC and CO.
However, the three-way catalyst works when the engine is operating in the stoichiometric (air-fuel ratio feedback) mode but does not sufficiently work in the lean mode (excess of O2 state) or in the rich mode (lack of O2 state).
In recent years, in particular, the operation in the lean mode has been widely adopted from the standpoint of improving fuel efficiency, and it has been urged to efficiently purify the exhaust gas in the lean operation condition.
In order to purify the exhaust gases in the lean operation condition, therefore, it has been proposed to dispose a NOx occlusion-type catalyst (NOx occluding catalyst) on the down stream of the three-way catalyst in order to occlude NOx that was not all reduced by the three-way catalyst.
However, NOx occluded by the NOx occluding catalyst during the lean operating condition must be reduced with reducing agents (HC, CO). For this purpose, the engine operating condition after the occlusion of NOx must be forcibly controlled from the lean mode into the rich mode, resulting in a drop in the fuel efficiency.
In order to purify NOx in the lean mode, further, it has been proposed to use a selection-type NOx purifying catalyst (selectively reducing catalyst) in addition to using the three-way catalyst.
However, the selectively reducing catalyst works to purity NOx in the lean mode but its poor purifying efficiency is low, requiring much reducing agent for purifying NOx and without having a reducing agent-adsorbing function for adsorbing the reducing agent.
When the selectively reducing catalyst is installed on the downstream of the three-way catalyst, therefore, no NOx reducing agent flows to the downstream of the three-way catalyst in the lean mode. Due to the lack of the reducing agent, therefore, the selectively reducing catalyst fails to properly work, and it becomes difficult to reduce the amount of emission of NOx.
It further becomes necessary to provide a device (device for feeding atomized gas of fuel) for feeding a NOx reducing agent (HC) to the selectively reducing catalyst in the lean atmosphere. However, since the selectively reducing catalyst has no function for adsorbing the reducing agent, the NOx reducing agent must be continuously fed, requiring fuel in large amounts and deteriorating the fuel efficiency.
Besides, even if a reducing agent adsorbing member is added to the selectively reducing catalyst, most of the reducing agent that is adsorbed reacts with oxygen in the air. Accordingly, only the unreacted reducing agent is used for the purification of NOx; i.e., only a small proportion of the reducing agent is used.
When the selectively reducing catalyst is installed on the upstream of the three-way catalyst, on the other hand, the function for purifying the NOx is maintained by the selectively reducing catalyst in the lean mode. Immediately after the start of the engine (rich mode), however, an extended period of time is required until the three-way catalyst on the downstream side reaches an activating temperature. Thus, the three-way catalyst fails to work to a sufficient degree, and it becomes difficult to decrease the amounts of emission of HC and CO.
In order to pass the regulations on the exhaust gases that are becoming stringent in recent years, further, activation of the three-way catalyst must be promoted right after the start and, hence, the three-way catalyst must be placed just under the engine.
Therefore, the selectively reducing catalyst is inevitably installed on the downstream of the three-way catalyst, and the NOx reducing agents (HC, CO) necessary for the selectively reducing catalyst may not be sufficiently fed under the lean operating condition.
In the conventional device for purifying exhaust gas of an internal combustion engine as described above, use of the three-way catalyst only is not enough for purifying NOx to a sufficient degree in the lean operating condition. When the NOx occluding catalyst is used in combination, the rich operating condition must be repetitively established to reduce NOx that is occluded resulting in a decrease in the fuel efficiency.
When the selectively reducing catalyst is installed on the downstream of the three-way catalyst, on the other hand, it becomes necessary to use a device for feeding a reducing agent, since the selectively reducing catalyst has a poor purifying efficiency, resulting in a decrease in the fuel efficiency, too.
When the selectively reducing catalyst is installed on the upstream of the three-way catalyst, an extended period of time is required until the three-way catalyst reaches the activating temperature. At the start, therefore, the three-way catalyst cannot exhibit its function of purification.
The present invention was accomplished in order to solve the above-mentioned problems, and has an object of providing a device for purifying exhaust gas of an internal combustion engine which features an improved purifying efficiency even in a lean operating condition without deteriorating the fuel efficiency, as a result of installing an electrochemical catalyst on the downstream of the three-way catalyst.
A device for purifying exhaust gas of an internal combustion according to the present invention comprises an electrochemical catalyst installed in the exhaust system of the internal combustion engine, the electrochemical catalyst containing an electron conducting substance and an ion conducting substance, the oxidizing reaction and the reducing reaction being promoted by the conduction of ions and electrons, thereby to electrochemically purify the exhaust gas in the exhaust system.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, the electrochemical catalyst contains at least either a NOx occluding member or an HC adsorbing member.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, further, a reducing agent feeding device is installed in the exhaust system of the internal combustion engine just on the upstream of the electrochemical catalyst to feed a NOx reducing agent into the exhaust system.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, the reducing agent feeding device feeds an atomized gas of fuel from the fuel tank as the NOx reducing agent.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, the reducing agent feeding device includes:
a canister for adsorbing the atomized gas of fuel; and
a reducing agent feed control valve for feeding the atomized gas of fuel adsorbed by the canister to the exhaust system of the internal combustion engine as the NOx reducing agent; and wherein,
the time for opening the reducing agent feed control valve is controlled depending upon the amount of the NOx reducing agent required by the electrochemical catalyst.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, the reducing agent feeding device feeds the fuel in the fuel tank as the NOx reducing agent.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, the reducing agent feeding device includes:
a fuel pump for feeding the fuel in the fuel tank while adjusting the pressure thereof to assume a predetermined value; and
a reducing agent feeding injector for feeding the fuel from the fuel pump as the NOx reducing agent into the exhaust system of the internal combustion engine; wherein,
the time for driving the reducing agent feeding injector is controlled depending upon the amount of the NOx reducing agent required by the electrochemical catalyst.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, the reducing agent feeding device feeds the fuel after reformed as the NOx reducing agent.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, the reducing agent feeding device includes:
a fuel pump for feeding the fuel in the fuel tank while adjusting the pressure thereof to assume a predetermined value;
a fuel reforming device for reforming the fuel fed from the fuel pump into H2; and
a feed amount control device for feeding H2 as the NOx reducing agent into the exhaust system of the internal combustion engine; wherein,
the feeding amount of the NOx reducing agent is controlled by the feed amount control device depending upon the amount of the NOx reducing agent required by the electrochemical catalyst.
Further, a device for purifying exhaust gas of an internal combustion engine according to the present invention comprises:
exhaust gas amount estimating means for estimating the amount of the exhaust gas containing NOx, HC and CO exhausted into the exhaust system of the internal combustion engine;
electrochemical catalyst state judging means for judging the active state of the electrochemical catalyst based upon the temperature data of the electrochemical catalyst; and
reducing agent feed judging means for estimating the reacting state of the electrochemical catalyst with the NOx relying upon the active state of the electrochemical catalyst, reaction rate thereof and storage of the exhaust gas by the electrochemical catalyst, and for judging whether the NOx reducing agent needs be fed based upon the values estimated by the exhaust gas amount estimating means;
wherein when it is so judged that the NOx reducing agent needs be fed, the reducing agent feed judging means drives the reducing agent feeding device.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, provision is made of sensor means for detecting at least one operating condition out of the rotational speed of the internal combustion engine, load condition and cooling water temperature, and wherein the exhaust gas amount estimating means estimates the amount of the exhaust gas depending upon the operating condition of the internal combustion engine.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, provision is made of first and second three-way catalysts in the exhaust system of the internal combustion engine, the first three-way catalyst being installed on the upstream of the electrochemical catalyst and the second three-way catalyst being installed on the downstream of the electrochemical catalyst.
In the device for purifying exhaust gas of an internal combustion engine according to the present invention, the internal combustion engine is controlled for its operation in a lean mode.